Hardy-Weinberg Equilibrium
Explore the Hardy-Weinberg equilibrium model to predict allele and genotype frequencies in non-evolving populations. Use the equations p² + 2pq + q² = 1 and p + q = 1 to calculate frequencies, and understand how violations of the five conditions (no mutation, random mating, no gene flow, infinite population size, no selection) indicate evolutionary change.
THE HARDY-WEINBERG EQUILIBRIUM
The **Hardy-Weinberg Principle** is a null model used to determine if a population is evolving. It states that in the absence of evolutionary forces, allele and genotype frequencies will remain constant from one generation to the next. If the actual frequencies in a population deviate from the predicted H-W frequencies, the population is currently evolving.
THE FIVE CONDITIONS FOR EQUILIBRIUM
1. **No Mutations**: No new alleles are added. 2. **Random Mating**: Every individual has an equal chance to reproduce. 3. **No Natural Selection**: All genotypes have equal survival and fitness. 4. **Extremely Large Population**: No **genetic drift** (random chance events). 5. **No Gene Flow**: No migration in or out.
HOW TO USE THIS VISUALIZATION
1. **Set Initial Frequencies**: Drag the slider to set the initial 'p' (dominant) and 'q' (recessive) frequencies. 2. **Simulate a Generation**: Click 'Next Generation' to see the offspring frequencies. 3. **Introduce a Force**: Toggle on 'Genetic Drift' (Small Population) or 'Selection' to watch the frequencies shift away from equilibrium. **Try This**: Set a small population size (N=10) and run 50 generations. Observe how one allele can completely disappear purely by chance. This is **fixation** via genetic drift.
CORE FORMULAS
AP EXAM CONNECTION
Unit: Unit 7: Natural Selection (Topic 7.5)
Learning Objective: EVO-1.K
COMMON MISCONCEPTIONS
- Thinking dominant alleles always increase in frequency (they stay constant unless selected for).
- Confusing p^2 (genotype) with p (allele).
- Assuming any change in frequency is always natural selection (it could be drift).
KEY TAKEAWAYS
- H-W is a baseline (null) model.
- Deviation from p^2 + 2pq + q^2 = Evolution.
- Drift is powerful in small populations.
- Selection changes allele frequencies.
PRACTICE QUESTIONS
Q1 (QUANTITATIVE): A population has a recessive phenotype frequency (q^2) of 0.04. What is the frequency of the recessive allele (q)?
Show Answer & Explanation
Answer: 0.2.
Explanation: If , then is the square root of 0.04, which is 0.2.
Q2 (QUANTITATIVE): A population of 100 individuals has 64 homozygous dominant (AA) individuals. If it is in equilibrium, what is the frequency of the heterozygous (Aa) genotype?
Show Answer & Explanation
Answer: 0.32.
Explanation: If , then . Since , . The frequency of heterozygotes is .
